1. Field of the Invention
This invention relates to an optical system for detecting a horizontal synchronizing signal for determining the start timing of a laser output for printing, drawing, etc. in an optical scanning apparatus of a laser printer, etc.
2. Description of the Prior Art
An optical scanning apparatus of a laser printer, etc. has a light receiving element for receiving light outside an effective scanning range for using a light flux from a scanning lens for drawing in order to obtain a good timing of a starting point for drawing, and a horizontal synchronizing signal a so-called BD pulse, is output from this light receiving element.
FIG. 9 shows an optical system of a conventional optical scanning apparatus. A light flux coming from a light source (not shown) is reflected and deflected by a polygon mirror 1 and focused onto an objective surface to be scanned (not shown) through an f.theta. lens 2. A mirror 3 is disposed outside the effective scanning range and adapted to reflect the light flux from the f.theta. lens toward a light receiving element 4 which is adapted to detect a horizontal synchronizing signal.
In order to make the rising of the output sharp, the light receiving element 4 is disposed in the vicinity of a position where the light flux is most converged, that is, in the vicinity of a position equivalent to an image surface.
However, with the construction of FIG. 9, the optical path from the f.theta. lens 2 to the light receiving element 4 is too long and the optical system can not be made compact. Therefore, various proposals have heretofore been made in order to shorten the optical path and make the optical apparatus compact.
However, by merely placing the light receiving element nearer to the f.theta. lens it is impossible to achieve the intended result because the diameter of the light flux, which is made incident to the light receiving element, becomes large and the bottom portion of the distribution of light quantity spreads out which eventually makes the rising of the output dull (i.e., not sharp). In order to prevent this, it is necessary to keep the construction such that the light receiving element is disposed where the light flux is most converged.
Japanese Patent Early Laid-open Publication Nos. Sho 62-75612, 63-98624 and 63-173010 disclose providing a convex lens and a concave mirror in an optical path between a scanning lens and a light receiving element in order to shorten the length of the optical path.
However, the employment of the convex lens and the concave mirror in order to achieve the object of making the apparatus compact gives rise to the following problems.
First, as the power of the f.theta. lens is added to the power of the convex lens, etc., and angle of convergence of the light flux becomes large (i.e., F number becomes small) when compared with a case where the convex lens, etc. is not utilized. Therefore, even a slight displacement of the light receiving element makes the diameter of the light flux large, and the rising of the output becomes dull. In order to prevent this, the accuracy required for mounting the light receiving element becomes critical.
Second, due to the power of the convex lens, etc., scanning speed of the light flux becomes slow and the rising of the output of the light receiving element becomes dull.
In case where the output is dull, an accuracy of determining the start point of the drawing gets worse.